Results 1 - 10 of 6311
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[en] A 'weighted magnetic bearing' has been developed to improve the performance of rotating Lorentz-force flowmeters (RLFFs). Experiments have shown that the new bearing reduces frictional losses within a double-sided, disc-style RLFF to negligible levels. Operating such an RLFF under 'frictionless' conditions provides two major benefits. First, the steady-state velocity of the RLFF magnets matches the average velocity of the flowing liquid at low flow rates. This enables an RLFF to make accurate volumetric flow measurements without any calibration or prior knowledge of the fluid properties. Second, due to minimized frictional losses, an RLFF is able to measure low flow rates that cannot be detected when conventional, high-friction bearings are used. As a result, this paper provides a brief background on RLFFs, gives a detailed description of weighted magnetic bearings, and compares experimental RLFF data to measurements taken with a commercially available flowmeter.
[en] Here, optically pumped magnetometers (OPM) can be used in various applications, from magnetoencephalography to magnetic resonance imaging and nuclear quadrupole resonance (NQR). OPMs provide high sensitivity and have the significant advantage of non-cryogenic operation. To date, many magnetometers have been demonstrated with sensitivity close to 1 fT, but most devices are not commercialized. Most recently, QuSpin developed a model of OPM that is low cost, high sensitivity, and convenient for users, which operates in a single-beam configuration. Here we developed a theory of single-beam (or parallel two-beam) magnetometers and showed that it is possible to achieve good sensitivity beyond their usual frequency range by tuning the magnetic field. Experimentally we have tested and optimized a QuSpin OPM for operation in the frequency range from DC to 1.7 kHz, and found that the performance was only slightly inferior despite the expected decrease due to deviation from the spin-exchange relaxation-free regime.
[en] Atomic force microscopy (AFM) experiments were performed to study the behaviour of AFM cantilevers under an external magnetic field B and temperature field produced by a coil with an iron core. Four cantilever types were studied. Forces were measured for different B values and at various coil-to-cantilever separation distances. The results were analysed on the basis of a phenomenological model. This model contains the contribution of two terms, one monopole-monopole interaction at short distance, and one apparent paramagnetic interaction in ∇B2 at large distance, which represents the temperature effects. We observe a good agreement between the model and the experimental data. (authors)
[en] The basic design and most relevant experimental conditions of a pulsed metastable atomic-beam oven are described. The stainless steel oven is suitable for vaporising metals and salts up to around 1400 K producing intense beams of metastable alkaline-earth atoms when pulsed or continuous wave low voltage discharges are used. Several applications using atomic calcium in its 3P and 1D electronic state are reported. The beam characterisation and discharge efficiency have been measured by time-of-flight or laser-induced fluorescence techniques. In addition, a method of changing the metastable n3P/n1D ratio, by raising the oven temperature, is described which looks very promising for the study of electronic selectivity in reactive collision processes. Finally several spectroscopic applications for atomic and molecular beam determinations are reported. (author)
[en] We describe the space-resolved soft x-ray (1-33nm) instrumentation developed for the Tore Supra tokamak. By using a programmable hydraulic jack to move the spectrometer, several spatial profiles (up to ten) of many impurity lines are obtained during a single plasma discharge, with a time resolution which can be as short as 600 ms. (author)
[en] A simple one-dimensional position-sensitive detector using delay-line encoding to achieve deadtimes below 100 ns for groups of particles is presented. The design allows for trade-off between positional resolution, deadtime and the number of particles per group. The prototype performance ( 1mm resolution, 100 ns deadtime for up to 4 particles) has been tested in applications to dissociative single and double photoionisation of small molecules. The forward/backward electron ejection asymmetry in NO photoionisation to NO+ (c3Π) and an axial/transverse asymmetry on O2 photoionisation are confirmed. (author)
[en] This paper presents a new method performing high-quality 3D object reconstruction of complex shapes derived from multiple, calibrated photographs of the same scene. The novelty of this research is found in two basic elements, namely: (i) a novel voxel dissimilarity measure, which accommodates the elimination of the lighting variations of the models and (ii) the use of an ant colony approach for further refinement of the final 3D models. The proposed reconstruction procedure employs a volumetric method based on a novel projection test for the production of a visual hull. While the presented algorithm shares certain aspects with the space carving algorithm, it is, nevertheless, first enhanced with the lightness compensating image comparison method, and then refined using ant colony optimization. The algorithm is fast, computationally simple and results in accurate representations of the input scenes. In addition, compared to previous publications, the particular nature of the proposed algorithm allows accurate 3D volumetric measurements under demanding lighting environmental conditions, due to the fact that it can cope with uneven light scenes, resulting from the characteristics of the voxel dissimilarity measure applied. Besides, the intelligent behavior of the ant colony framework provides the opportunity to formulate the process as a combinatorial optimization problem, which can then be solved by means of a colony of cooperating artificial ants, resulting in very promising results. The method is validated with several real datasets, along with qualitative comparisons with other state-of-the-art 3D reconstruction techniques, following the Middlebury benchmark. (paper)
[en] Airborne acoustic imaging has the capability of obtaining distance information of an object in a scene, the capability of distinguishing objects from the background or a texture and it can also estimate the velocity of objects. However, the nonideal radiation pattern of a transmitter used for insonifying an imaging space can degrade imaging results. In this work, radiation patterns have been investigated, and a new approach to decrease the effect of a transmitter’s radiation pattern in acoustic imaging is proposed. The novelty of our proposed method is that we used the measurement-based transmitter’s radiation compensation. The compensation can be accomplished without knowing anything about the model and characteristics of the transmitter, the receiver and the medium. We compensated for the transmitter’s radiation pattern and reconstructed acoustic images using the synthetic transmit aperture imaging technique. The compensation was based on the radiation pattern obtained from real measurements using a Wiener filter. The Wiener filter was used to compensate for the nonideal radiation pattern of the ultrasonic transmitter for both the phase and amplitude aspects simultaneously. To verify the proposed method, an indoor airborne acoustic imaging experiment was conducted using a two-dimensional (2D) receiver array and a 2D transmitter array. The results show an obvious improvement in the vertical angular resolution of the reconstructed three-dimensional images as well as a satisfactory horizontal angular resolution. (paper)
[en] This paper reports a novel and versatile system for measuring excess noise and multiplication in avalanche photodiodes (APDs), using a bipolar junction transistor based transimpedance amplifier front-end and based on phase-sensitive detection, which permits accurate measurement in the presence of a high dark current. The system can reliably measure the excess noise factor of devices with capacitance up to 5 nF. This system has been used to measure thin, large area Si pin APDs and the resulting data are in good agreement with measurements of the same devices obtained from a different noise measurement system which will be reported separately. (paper)
[en] Wet gas metering with differential pressure (DP) devices (e.g. the orifice plate, the Venturi and the V-Cone) has gained increasing interest in the oil and gas industry. Many investigations have been performed and several models have been proposed. Among the DP devices, the V-Cone flow meter has received increasing attention owing to its remarkable performance characteristics, including high accuracy, excellent repeatability, wide turndowns, shorter straight length and stable signals. In this work, we developed a new method for predicting the gas flow rate in low pressure wet gas flow using a V-Cone flow meter with the diameter ratio of 0.55. The experimental fluid was air and tap water. The test pressure ranged from 0.10 to 0.48 MPa, and the gas and liquid mass flow rates ranged from 100 to 500 N m3 h−1 and from 0.030 to 0.358 m3 h−1, respectively. Thus, the Lockhart–Martinelli parameter, XLM, was up to 0.158 and the gas volume fraction ranged from 98.94% to 100%. A dimensionless parameter, K, was proposed in this work and defined as the two-phase flow coefficient of the flow meter. The results indicated that the K linearly increased with the Lockhart–Martinelli parameter. In addition, the K increased with the gas densiometric Froude number and decreased with the operating pressure when other parameters were kept constant. On the basis of the two-phase flow coefficient, a new wet gas model was developed and compared with seven popular wet gas models. It was found that with the V-Cone flow meter and under the present experimental conditions the new model produced a more accurate prediction of the wet gas than other models. The research approach to obtaining the model can also be used in the studies on other DP devices and thus will benefit the design of wet gas meters. (paper)